A conventional boiling water reactor includes a pressure vessel containing a reactor core and water, the core being effective for boiling the water for generating steam at a relatively high pressure. The steam is conventionally channeled to a conventional steam turbine for extracting energy therefrom and for driving, for example, a conventional electrical generator for generating power for a utility grid. The nuclear reactions, or reactivity, in the reactor core are conventionally controlled by conventional control rods. The control rods may be selectively inserted into the reactor core to decrease reactivity and to shut down the reactor core.
The reactor also typically includes an additional reactivity control system as a secondary, or backup reactivity control system in the event the primary system fails to achieve shutdown. In one exemplary alternate reactivity control system, relatively large holding tanks of conventional nuclear poison, such as boron-containing-water solutions, are provided adjacent to the reactor in the containment building. When activated, conventional positive displacement pumps inject the nuclear poison solution into the reactor vessel wherein it is mixed with the recirculating water therein to shut down the reactor core. The pump must be capable of generating sufficient flow rates and relatively high pressures of about 1,500 psi (10.3 MPa) for promptly channeling the nuclear poison into the reactor vessel which is maintained at a pressure of about 1,200 psi (8.3 MPa), for example. A conventional explosive valve is provided between the pump and the reactor vessel and is normally closed. Once the pump establishes an adequate pressure greater than that in the reactor vessel, the explosive valve is conventionally opened to allow the nuclear poison solution to quickly enter the reactor vessel.
Concepts also exist for obtaining a passive system without the use of pumps for effectively providing the nuclear poison to the reactor vessel. In one concept, the relatively large accumulator filled with the nuclear poison solution is pressurized by a gas to an overpressure greater than that within the reactor vessel. When required, a conventional explosive valve is opened to release the nuclear poison from the accumulator and into the reactor vessel. However, maintaining a large high pressure accumulator within the containment building poses a concern since adequate safety provisions must be provided.